Patent CH-A-118 038 describes a watch case forming an outer envelope intended to receive a movement. Such outer envelope is made up of a back cover/caseband and of a bezel. In order to avoid the usual arrangement of a screw thread coupling them together, the use of a bayonet joint has been conceived in order to be able, during winding of the movement, to separate and reunite the back cover/caseband with the bezel without difficulty and rapidly. In order to attain this purpose, the bezel bears studs intended to fit into grooves formed in the back cover/caseband. This latter bears springs which project to the internal end of the grooves and behind which are engaged the studs having come to the end of the travel in such grooves. In this construction, it is sufficient to give the bezel an inverse rotational movement in order to disengage the bezel from the back cover/caseband. This system thus does not exhibit the safety arrangement preventing an untimely rotational movement which could accidentally come to free the movement.